This invention relates in general to the synthesis of highly pure, polycrystalline, indium phosphide. In a more particular aspect, this invention concerns itself with a method for the in-situ baking of raw indium to produce indium of the highest quality prior to its use as one of the reaction components in the synthesis of polycrystalline indium phosphide.
High purity, single crystal, indium phosphide, a group III-V semiconductor compound, has become an important semiconductor material for a number of technical applications. It is considered to be a critical material for use as a substrate for lattice-matched fiber-optic sources and detectors, high speed integrated circuits, and high frequency microwave devices. Its large band gap (1.35 eV) and high electron mobility make it useful as a semiconductor material, especially when employed in its highly pure form and as a crystalline substrate for device fabrication by epitaxial deposition techniques. However, it is extremely difficult to grow single crystals of indium phosphide having the degree of purity necessary for its successful utilization as a thin film device.
A number of methods have been suggested for synthesizing polycrystalline indium phosphide. One method for synthesizing fairly large ingots involves the direct reaction of elemental phosphorus with elemental indium. This method offers the advantgage of producing a relatively pure product since there is no possibility of contamination by other reactants. Unfortunately, the reaction often leads to large pressure buildup with a resulting explosive potential. This method requires small, strongly sealed containers or bombs and is an expensive and dangerous method for conducting the reaction. Also, the indium and phosphorus reaction components, in their raw form, do not possess a basic purity requisite to the ultimate synthesis of a highly pure indium phosphide polycrystalline ingots. Reaction methods involving compounds of indium and phosphorus, rather than elemental reactants, have also been suggested, but the resulting indium phosphide has often been lacking in the degree of purity needed to grow good single crystals.
A method which has proven successful in growing single crystal InP is the Liquid Encapsulated Czochralski (LEC) technique from polycrystalline InP. The polycrystalline ingots of indium phosphide are generally synthesized from solution in a modified Bridgman apparatus. Unfortunately, however, the polycrystalline indium phosphide feed material does not possess the very high degree of purity needed for producing InP single crystals having the necessary electrical properties which lead to the efficient utilization of InP semiconductor materials.
In an attempt to overcome this problem, it was found that the in-situ vacuum baking of a raw indium reaction component immediately prior to synthesizing a polycrystalline material ingot would lead to the growth of single crystal materials with the highest purity and lowest residual carrier concentration.